Method of forming dislocation-free strained thin films
A method of forming a stressed thin film on a substrate includes forming a plurality of islands on a viscous layer that is present on a surface of a substrate. Adjacent islands are bridged with a stressor layer. The structure is annealed at an elevated temperature above the glass flow temperature of the viscous layer to transfer at least a portion of the stress from the stressor layer to the underlying islands. The bridges are then removed to expose the stressed islands of thin film on the substrate.
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This Application claims priority to U.S. Provisional Patent Application No. 60/700,449 filed on Jul. 19, 2005. U.S. Provisional Patent Application No. 60/700,449 is incorporated by reference as if set forth fully herein.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENTThis invention was made with Government support under Grant No. FA9550-04-1-0370, awarded by the Air Force Office of Scientific Research. The Government has certain rights in this invention.
FIELD OF THE INVENTIONThe field of the invention generally relates to methods for forming stressed (e.g., compressive of tensile) thin films. More particularly, the field of the invention relates to methods used to form dislocation-free stressed thin films.
BACKGROUND OF THE INVENTIONThe use of strained silicon devices is known to increase semiconductor device performance. For example, in the context of transistors, strained silicon increases the transistor drive current which improves switching speed by making current flow more smoothly. Generally, a very thin layer of single-crystal silicon with built in stress (or strain) improves drive current making the devices fun faster. When the layer of silicon is under stress, the silicon lattice lets electrons and holes flow with less resistance. For transistors, the lower resistance translates in to faster switching properties, thereby permitting semiconductor devices to operate at faster speeds.
Because of the advantages inherent in the strained lattice structure, strained silicon or silicon germanium based devices have become an attractive alternative to current microelectronic devices that are composed of a silicon channel layer on a silicon substrate. Several approaches have been developed to form strained silicon on substrates. For example, relaxed silicon germanium buffer layers have been employed as a “virtual substrate” to grow strained silicon. Typically, the relaxed silicon germanium buffer layer, which has a higher lattice constant than the silicon substrate, is formed in a graded manner and is used as an epitaxial growth template.
If a constant (i.e., non-graded composition) silicon germanium buffer layer is used, high densities of dislocations nucleate during growth and interact with one another. This interaction prevents dislocations from propagating to the edge of the substrate (e.g., a wafer), thereby leaving a significant number of threading arms on the surface of the silicon germanium layer. In contrast, by grading the germanium composition during growth of the relaxed silicon germanium layer on a silicon substrate, the nucleation rate of dislocations is retarded by reducing the strain accumulation rate. Consequently, the interaction between dislocations is reduced, significantly reducing the density of threading arm dislocations on the surface of the silicon germanium layer. For example, the threading dislocation density in a constant (non-graded) silicon germanium grown directly on a silicon substrate is on the order of about 108˜9/cm2. If a graded silicon germanium buffer layer is formed on a silicon substrate, the threading dislocation density improves to around 104˜5 cm2.
Unfortunately, there are several disadvantages to graded silicon germanium buffer layers. First, the threading dislocation density, while lower in graded buffer layers, is still non-zero, which leads to degradation of electron and hole mobility. Moreover, a large thickness of graded silicon germanium buffer layer is needed for achieving low threading dislocation densities. The large thickness increases the size of the devices as well as the cost of production. Second, the strain-relaxed graded silicon germanium buffer layer has a rough surface which degrades the mobility of strained silicon. In addition, the strain at the top layer of silicon is not homogeneous due to the stress fields from buried dislocations, which also adversely affects carrier transport.
Attempts have also been made to form relaxed, dislocation free buffer templates of silicon germanium on viscous glass layers. For example, Hobart et al. disclose a process of relaxing compressively strained heteroepitaxial silicon germanium films bonded to a viscous borophosphosilicate glass (BPSG) film. Hobart et al., Compliant Substrates: A Comparative Study of the Relaxation Mechanisms of Strained Films Bonded to High and Low Viscosity Oxides, Journal of Electronic Materials, Vol. 29, No. 7, 2000. In this method, compressed silicon germanium thin film “islands” are transferred to a viscous BPSG layer by wafer bonding. The metastable compressed silicon germanium islands start to undergo elastic relaxation at heating of near 800° C. Because of the island patterning, the small silicon germanium islands may allow faster relaxation than dislocation introduction. Unfortunately, using this method, dislocations will develop if the thickness of the silicon germanium is too large. Typically, the maximum strain in a dislocation-free film that can be produced is around 1% if the film is kept thinner than about 10 nm. On the other hand, if the film is too thin, the relaxation process may introduce wrinkles into the silicon germanium film.
SUMMARY OF THE INVENTIONIn a first aspect of the invention, a method of forming a stressed thin film on a substrate includes forming a plurality of thin film islands on a viscous layer formed on a substrate. The thin film islands may be formed from silicon or a type III-V semiconductor. Adjacent thin films are bridged with a stressor layer. The structure is annealed at an elevated temperature above the glass flow temperature of the viscous layer. The bridges are then removed to expose the stressed thin film on the substrate.
In another aspect of the invention, a method of forming a stressed thin film on a substrate includes the steps of forming a plurality of islands on a viscous layer formed on substrate. A second substrate is provided having a stressor layer formed thereon. The stressor layer (on the second substrate) is then bonded to the plurality of islands. The second substrate is then removed, for example, through a SMART-CUT process. The stressor layer is then patterned to form bridges between at least a portion of the adjacent islands. The structure is then heated at an elevated temperature above the glass flow temperature of the viscous layer. The bridges are then removed so as to expose the stressed thin film islands.
In another aspect of the invention, a method of forming a stressed thin film on a substrate includes the steps of forming a plurality of islands on a viscous layer formed on a substrate, each island having a stressor layer disposed thereon. The structure is then subject to annealing temperatures above the glass flow temperature of the viscous layer. The stressor layer is removed so as to expose a plurality of stressed, thin film islands. Another stressor layer is deposited on the plurality of stressed thin film islands and the structure is then subject to annealing temperatures above the glass flow temperature of the viscous layer. The stressor layer is then removed so as to expose a plurality of stressed thin film islands. The process can be repeated with additional stressor layers to customize the level of stress in the thin film islands.
Still referring to
The thin film of silicon 14 may have a thickness that is generally less than 500 Å. The thickness of the thin film of silicon 14 may vary depending on the applied stress from the overlying stressor layer (discussed in more detail below). Generally, the thickness of the thin film of silicon 14 is kept less than the critical thickness at which dislocations are induced or otherwise generated at a given stress level. As an alternative to silicon, the thin film 14 may be formed from other type III-V semiconductor materials.
The stressor layer 16 may be formed from materials other than those specifically mentioned above. For example, the stressor layer 16 may be formed from metals (e.g., nickel), semiconductors, and dielectric materials known to produce large degrees of stress (either compressive or tensile). Generally, stress in the stressor layer 16 may be formed as a result of the deposition process (i.e., residual strain) or via thermal stress caused by expansion of the material under elevated temperatures. Nickel, for example, has a high thermal expansion coefficient and a high melting point. Therefore, nickel produces very high stress at annealing temperatures of BPSG (˜800° C.) without significant softening due to its high melting point.
The thickness of the stressor layer 16 may vary over a wide range, for example, from microns to millimeters. Unlike the prior art methods disclosed in Hobart et al., there is no size restriction on the stressor layer 16 eliminating the chance of film wrinkling. The stressor layer 16 formed on the thin film of silicon 14 forms a bilayer structure as is shown in
Referring now to
Turning now to
After relaxation, as shown in
The method described with respect to
In one alternative aspect of the invention, the process described above is repeated a plurality of times to achieve the desired level of strain in the stressed islands 20. For example, if a single annealing and relaxation process does not transfer enough stress to the stressed islands, an additional stressor layer 16 may be deposited over the stressed islands 20. The additional stressor layers 16 may be applied using a separate substrate containing the stressor layer 16 such as that disclosed in the method illustrated in
As shown in
Next, as shown in
Next, as shown in
With reference to
An advantage of the method illustrated in
An additional advantage of the methods described herein is that the stressed islands 14, 54 are formed substantially if not entirely free of defects. In additional, since germanium is not used (in the exemplary embodiments) the problem of rampant inter-diffusion between silicon and germanium that is common in high temperature processes and self-heating from silicon germanium is avoided.
While embodiments of the present invention have been shown and described, various modifications may be made without departing from the scope of the present invention. The invention, therefore, should not be limited, except to the following claims, and their equivalents.
Claims
1. A method of forming a stressed thin film on a substrate comprising:
- forming a plurality of thin film islands on a viscous layer formed on a substrate;
- bridging adjacent thin film islands with a stressor layer, the stressor layer suspended above the viscous layer;
- annealing the viscous layer above its glass flow temperature; and
- removing the bridges.
2. The method of claim 1, wherein the stressor layer is under compressive stress.
3. The method of claim 1, wherein the stressor layer is under tensile stress.
4. The method of claim 1, wherein the viscous layer comprises borophosphosilicate glass.
5. The method of claim 1, wherein the stressor layers comprises silicon dioxide, silicon nitride, tungsten silicide, and nickel.
6. The method of claim 1, wherein the thin film islands are substantially free from dislocations.
7. The method of claim 1, wherein a single bridge connects two adjacent silicon thin film islands.
8. A method of forming a stressed thin film on a substrate comprising:
- forming a plurality of islands on a viscous layer formed on a substrate;
- providing a second substrate having a stressor layer formed thereon;
- bonding the stressor layer of the second substrate to the plurality of islands;
- removing the second substrate;
- patterning the stressor layer to form bridges between at least a portion of adjacent islands;
- annealing the viscous layer above its glass flow temperature; and
- removing the bridges so as to expose stressed thin film islands.
9. The method of claim 8, wherein the stressor layer is under compressive stress.
10. The method of claim 8, wherein the stressor layer is under tensile stress.
11. The method of claim 8, wherein the viscous layer comprises borophosphosilicate glass.
12. The method of claim 8, wherein the bridges are suspended above the viscous layer.
13. The method of claim 8, wherein the bridges are in contact with the viscous layer.
14. The method of claim 8, wherein the stressed thin film islands are substantially free from dislocations.
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Type: Grant
Filed: Jul 19, 2006
Date of Patent: Jul 13, 2010
Patent Publication Number: 20070017438
Assignee: The Regents of the University of California (Oakland, CA)
Inventors: Ya-Hong Xie (Beverly Hills, CA), Jeehwan Kim (Los Angeles, CA)
Primary Examiner: Robert M Kunemund
Assistant Examiner: Matthew J Song
Attorney: Vista IP Law Group LLP
Application Number: 11/458,628
International Classification: H01L 21/324 (20060101);